Multi-rotor noise control by automated distribution propulsion

ABSTRACT

A method of reducing noise generated by a tilt-rotor aircraft includes transitioning the tilt-rotor aircraft into an airplane mode from a helicopter mode, and reducing a speed of a first pair of fans of the tilt-rotor aircraft to be less than a speed of a second pair of fans that are positioned in-line with the first pair of fans. A flight control system configured to reduce a noise level of a tilt-rotor aircraft includes a flight control computer comprising a processor, a propulsion system communicatively coupled to the flight control computer, a first pair of fans and a second pair of fans communicatively coupled with the flight control computer and the propulsion system. The processor is operable to implement a method that includes transitioning the tilt-rotor aircraft into an airplane mode from a helicopter mode, and reducing a speed of the first pair of fans to be less than a speed of the second pair of fans that are positioned in-line with the first pair of fans.

TECHNICAL FIELD

The present disclosure relates generally to rotor-driven aircraft andmore particularly, but not by way of limitation, to reduction of noisegenerated by rotor-driven aircraft.

BACKGROUND

This section provides background information to facilitate a betterunderstanding of the various aspects of the disclosure. It should beunderstood that the statements in this section of this document are tobe read in this light, and not as admissions of prior art.

Rapid commercial growth and expansion of urban areas often increases thedistance from one side of a metropolitan area to another. This rapidcommercial growth and expansion often results in an increase in thepopulation, further resulting in more congestion and emissions due to anincreased number of vehicles on the current highway infrastructure. Astechnology further increases, such metropolitan areas will continue togrow, placing serious burden on the current highway infrastructure tohandle the increased traffic and furthering the need for improved travelacross a metropolitan area that reduces emissions while allowing faster,more convenient, and more efficient travel throughout a metropolitanarea and/or between bordering states.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it to be used as an aid in limiting the scope of theclaimed subject matter.

An illustrative method of reducing noise generated by a tilt-rotoraircraft includes transitioning the tilt-rotor aircraft into an airplanemode from a helicopter mode, and reducing a speed of a first pair offans of the tilt-rotor aircraft to be less than a speed of a second pairof fans that are positioned in-line with the first pair of fans.

An illustrative flight control system configured to reduce a noise levelof a tilt-rotor aircraft includes a flight control computer comprising aprocessor, a propulsion system communicatively coupled to the flightcontrol computer, a first pair of fans and a second pair of fanscommunicatively coupled with the flight control computer and thepropulsion system. The processor is operable to implement a method thatincludes transitioning the tilt-rotor aircraft into an airplane modefrom a helicopter mode, and reducing a speed of the first pair of fansto be less than a speed of the second pair of fans that are positionedin-line with the first pair of fans.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a perspective view of an aircraft oriented in a helicoptermode according to aspects of the disclosure;

FIG. 2 is a perspective view of an aircraft oriented in an airplane modeaccording to aspects of the disclosure;

FIG. 3 is a schematic diagram of the aircraft of FIGS. 1 and 2 accordingto aspects of the disclosure;

FIGS. 4A and 4B are flow diagrams illustrating methods of reducing noiseproduced by an aircraft according to aspects of the disclosure; and

FIG. 5 is a schematic diagram of a general-purpose processor (e.g.electronic controller or computer) system suitable for implementingaspects of the disclosure.

DETAILED DESCRIPTION

Various aspects will now be described more fully with reference to theaccompanying drawings. The disclosure may, however, be embodied in manydifferent forms and should not be construed as limited to the aspectsset forth herein.

Referring now to FIGS. 1 and 2, perspective views of an aircraft 100operating in helicopter and airplane modes, respectively, are shownaccording to aspects of the disclosure. Aircraft 100 is generallyconfigured as a vertical takeoff and landing (VTOL) aircraft, morespecifically a tiltrotor, that is operable in an airplane modeassociated with forward flight and a helicopter mode associated withvertical takeoff from and landing to a landing zone. Aircraft 100comprises a fuselage 102, a cockpit and/or passenger compartment 104,wings 106 extending from the fuselage 102, vertical stabilizers 108, ahorizontal stabilizer 110, a pair of fans 112 carried by, supported byand/or otherwise coupled to fuselage 102, a pair of fans 114 carried by,supported by, and/or otherwise coupled to wings 106, and a pair of fans116 carried by, supported by, and/or otherwise coupled to fuselage 102.Fans 112, 114, 116 are arranged about fuselage 102 to be generallycoplanar when aircraft 100 is in helicopter and airplane mode. Aircraft100 also includes a flight control system 130 (e.g., see FIG. 3) thatincludes a flight control computer 140 and a propulsion system 150, bothof which are communicatively coupled with fans 112, 114, and 116.

The pair of fans 112 are supported by a rotatable shaft extending atleast partially through fuselage 102 and coupled to the pair of fans112. The pair of fans 112 may be selectively rotated with respect tofuselage 102 by at least one actuator (e.g. electric,electro-mechanical, magnetic, and/or hydraulic) in order to transitionaircraft 100 between the airplane mode and the helicopter mode. Each fan112 comprises a duct 118 having a plurality of structural supportsand/or struts 120. In some aspects, outer surfaces of the ducts 118 maybe shaped to provide optimal and/or preferred flight characteristics inat least one of the airplane mode and the helicopter mode. It will beappreciated that the rotor systems 122 of each fan 112 rotate inopposing directions with respect to one another to balance the torquegenerated by each fan 112.

Each fan of the pair of fans 112 comprises a single rotor system 122having a plurality of rotor blades 124 attached thereto. The rotorblades 124 are disposed within the duct 118 and configured to generatethrust when selectively rotated. As illustrated in FIG. 1, each rotorsystem 122 comprises four rotor blades 124. However, in other aspects,each rotor system 122 may comprise two, three, five, six, seven, eight,and/or more rotor blades 124.

Each wing 106 carries a single fan of the pair of fans 114. The pair offans 114 are supported by a rotatable shaft extending at least partiallythrough wings 106 and coupled to the pair of fans 114. The pair of fans114 may be selectively rotated with respect to fuselage 102 by at leastone actuator (e.g. electric, electro-mechanical, magnetic, and/orhydraulic) in order to transition aircraft 100 between the airplane modeand the helicopter mode. The pair of fans 114 are structurally similarto the pair of fans 112 and each fan of the pair of fans 114 includesits own duct 118, struts 120, single rotor system 122, and plurality ofrotor blades 124. Compared to the pair of fans 112, the pair of fans 114are disposed further outboard of fuselage 102.

The pair of fans 116 are supported by a rotatable shaft extending atleast partially through fuselage 102 and coupled to the pair of fans116. The pair of fans 116 may be selectively rotated with respect tofuselage 102 by at least one actuator (e.g. electric,electro-mechanical, magnetic, and/or hydraulic) in order to transitionaircraft 100 between the airplane mode and the helicopter mode. The pairof fans 116 are structurally similar to the pair of fans 112, 114 andeach fan of the pair of fans 116 includes its own duct 118, struts 120,single rotor system 122, and plurality of rotor blades 124. The pair offans 116 are generally disposed in-line with the pair of fans 112 whenaircraft 100 operates in airplane mode (e.g., see FIG. 2). As usedherein, “disposed in-line” is used to mean that a first pair of fans(e.g., the pair of fans 116) are positioned such that in airplane modethe first pair of fans ingest turbulent air generated by a second pairof fans (e.g., the pair of fans 112) or a portion of aircraft 100 (e.g.,fuselage, cockpit, passenger compartment, wings) positioned forward ofthe first pair of fans. In a typical aspect, “disposed in-line” meansthat, in airplane mode, an axis of rotation of a first pair of fans anda second pair of fans are co-linear or generally parallel to one anotherand offset less than one fan diameter. In a typical aspect, “disposedout of line” is used to mean that, in airplane mode, an axis of rotationof a first pair of fans and a second pair of fans are generally parallelto one another and offset more than one fan diameter.

Aircraft 100 is controlled via flight control system 130. Flight controlsystem 130 includes flight control computer 140 that connected to and incommunication with propulsion system 150. Flight control computer 140 isconfigured to selectively control the components of propulsion system150 to operate aircraft 100. Flight control computer 140 may include oneor more systems 300, each of which includes a processor 310 that isoperable to implement the methods disclosed herein. An illustrativesystem 300 is discussed in more detail relative to FIG. 5.

Flight control system 130 may include flight control input hardware(e.g. flight controls 142) configured to receive inputs and/or commandsfrom a pilot to control operation of the aircraft 100 and/or a pluralityof sensors and/or gauges configured to provide feedback regardingoperational characteristics of aircraft 100 to the flight controlcomputer 130. Additionally, flight control computer 140 may beconfigured to selectively control the operation, orientation, rotation,position, and/or rotational speed of the pairs of fans 112, 114, 116. Insome aspects, flight control system 130 may comprise fly-by-wirearchitecture for controlling aircraft 100. Additionally, in someaspects, flight control system 130 may be capable of optionally-pilotedoperation. Furthermore, in some aspects, flight control system 130 maycomprise collective pitch control for adjusting the pitch of rotorblades 124 and rotational speed control for individually adjusting arotational speed of rotor systems 122 of each of the pairs of fans 112,114, 116, without the need for cyclic control for controlling operationof aircraft 100.

Propulsion system 150 is controlled by flight control computer 140 andincludes components that assist with the flight of aircraft 100.Propulsion system 150 may generally include a hybrid electrical system,a hybrid hydraulic system and/or combinations thereof. Propulsion system150 comprises an internal combustion engine and/or auxiliary power unit(APU) 152, a drive unit 154 that includes at least one of a powergenerator and a hydraulic pump, a battery bank 156, a plurality ofconduits 158 and a plurality of motors 160 (see FIG. 3). APU 152 isconfigured to power drive unit 154. In some aspects, APU 152 may includea gas turbine. In some aspects, APU 152 may be configured to provide anappropriate amount of power based on the power demands of aircraft 100.

The drive unit 154 is configured to drive the plurality of motors 160,which may be, for example, electric motors or hydraulic motors. In someaspects, drive unit 154 may include a power generator and/or analternator configured to generate sufficient electrical current in orderto drive electric motors 160. In some aspects, drive unit 154 mayinclude a hydraulic pump. In some aspects, drive unit 154 may compriseboth a power generator and/or alternator and a hydraulic pump, where thepower generator provides power (separately and/or in conjunction withAPU 152) to the hydraulic pump in order to drive hydraulic motors 160.It will be appreciated that in the aspects of a hybrid hydraulicpropulsion system 150, the hydraulic pump is capable of producingsufficient fluid pressure, velocity, and/or mass flowrate to powerhydraulic motors 160.

Battery bank 156 may be recharged via drive unit 154 when drive unit 154includes a power generator and/or alternator. However, in some aspects,battery bank 156 may alternatively be recharged via a current-producingcomponent (e.g., an alternator) of APU 152. Battery bank 156 may includea single battery or series of batteries that make up the primary powersource and provide high voltage direct current (DC) power to power motor160. Additionally, in some aspects, battery bank 156 may comprise aseparate emergency battery configured to provide power if the primarybattery bank 156 is approaching a low energy state (e.g., from use or anunexpected drop in energy). Additionally, it will be appreciated thatbattery bank 156 may also provide power to other systems of aircraft 100including, but not limited to, flight control system 130.

The electrical power from drive unit 154 and/or the battery bank 156 maybe delivered to motors 160 through a plurality of conduits 158. Inaspects of a hybrid electrical propulsion system 150, the plurality ofconduits 158 may comprise electrical conduits (e.g. electricallyconductive wires, electrical busses, etc.). In aspects of a hybridhydraulic propulsion system 150, the plurality of conduits 158 maycomprise hydraulic fluid conduits, or alternatively, a combination ofelectrical conduits and hydraulic fluid conduits.

The plurality of motors 160 may be disposed in the hub of each fan orcoupled to the hub and/or rotor mast of each rotor system 122 ofaircraft 100. Each motor of the plurality of motors 160 is configured toprovide selective rotation of the associated rotor system 122 to propelaircraft 100. In some aspects, the plurality of motors 160 may be directdrive electric motors. In some aspects, the plurality of motors 160 maybe hydraulic motors.

As illustrated in FIGS. 1 and 2, the pair of fans 114 are positionedoutboard of the pair of fans 112, 116. Positioning the pair of fans 114in this way provides space for wings 106 and also moves the pair of fans114 out of downstream alignment with the pair of fans 112 when aircraft100 operates in the airplane mode (e.g., see FIG. 2). Positioning thepair of fans 114 out of downstream alignment with the pair of fans 112allows the pair of fans 114 to ingest cleaner (i.e., less turbulent)air. Ingesting cleaner air increases efficiency and performance of thepair of fans 114 and reduces an amount of noise created by the pair offans 114. For a given fan rpm, the noise levels generated by a fan areincreased when ingesting turbulent compared to the noise levelsgenerated when ingesting less turbulent/clean air.

In contrast to the pair of fans 114, the pair of fans 116 are positionedin-line with the pair of fans 112 when aircraft 100 operates in airplanemode (e.g., see FIG. 2). When operating in helicopter mode (e.g., seeFIG. 1), placement of fans 112, 114, and 116 is ideal for verticaltakeoff/landing and hovering. However, when operating in airplane mode,placement of fans 112, 114, and 116 is less optimal due to the axialalignment of the pair of fans 112 with the pair of fans 116. Duringairplane mode, the pair of fans 116 may ingest turbulent air exhaustedby the pair of fans 112 (or generated from air passing over thefuselage, cockpit, passenger compartment, wings, etc.). Ingestion ofturbulent air reduces efficiency and performance of the pair of fans 116and also increases noise generated by the pair of fans 116. In somesituations, the amount of noise generated by aircraft 100 is ofparticular concern. For example, commercial aircraft are subject tocertain operating parameters, including noise certification. To reducethe noise level output by the pair of fans 116, a speed of the pair offans 116 is reduced.

Referring now to FIG. 3, a schematic diagram of aircraft 100illustrating flight control system 130 is shown according to aspects ofthe disclosure. Flight control system 130 includes flight controlcomputer 140 and propulsion system 150. Propulsion system 150 includesan internal combustion engine and/or auxiliary power unit (APU) 152, adrive unit 154, a battery bank 156, and a plurality of conduits 158 thatpermit communication between and provide power to components of aircraft100. APU 152 is configured to power drive unit 154, which can include,for example, a power generator, a hydraulic pump, and the like. Driveunit 154 provides power (e.g., electric or hydraulic power) to motors160 to drive rotor systems 122 of fans 112, 114, 116. Flight controlcomputer 140 is configured to communicate with propulsion system 150 toindividually control the power supplied to each motor of the pluralityof motors 160. In some aspects, flight control system 130 includes asensor 162 that monitors noise.

Flight control system 130 also includes flight controls 142. Flightcontrols 142 can include various actuators, servos, and the like forcontrolling aircraft 100 during flight. For example, flight controls 142may include actuators to control flaps, ailerons, tilt of fans 112, 114,116, landing gear, and the like.

During VTOL, fans 112, 114, 116 are oriented in helicopter mode as shownin FIG. 1. Flight control computer 140 controls power to the pluralityof motors 160 to provide the needed control authority to maintaincontrol of aircraft 100 during takeoff and landing. Power to each motor160 is individually controlled to maintain desired orientation ofaircraft 100. To transition to airplane mode, flight control system 130rotates fans 112, 114, 116 into the orientation shown in FIG. 2.

When operating in airplane mode, power to fans 112, 114, 116 istypically reduced as airplane mode requires less fan speed to maintainflight as compared to helicopter mode. For example, helicopter mode mayrequire fan speed in the 80-90% range and airplane mode may only requirearound 60% fan speed. These values are illustrative and may vary. Inairplane mode, each motor 160 of fans 112, 114, 116 is traditionallyoperated at the same speed. It has been discovered that operating thepair of fans 116 at the same speed as the pair of fans 112 can increasean amount of noise generated by aircraft 100 during flight. The increasein noise is a result of the pair of fans 116 ingesting turbulent aircreated by the pair of fans 112 (or generated from air passing over thefuselage, cockpit, passenger compartment, wings, etc.) that are locatedupwind of the pair of fans 116. In order to reduce the amount of noisegenerated by the pair of fans 116, flight control computer 140 decreasespower to the motors 160 powering the pair of fans 116 to reduce a speedof rotor systems 122 of the pair of fans 116. In some aspects, flightcontrol computer 140 may increase the power to either or both of thepair of fans 112 and 114 to make up for the reduced thrust provided bythe pair of fans 116. This load balancing is an automated distributionof torque. For example, the pair of fans 112 and 114 operate at a firstspeed and the pair of fans 116 operate at a second speed that is slowerthan the first speed.

In another aspect, automated distribution of torque can be accomplishedby operating the pair of fans 112 at a first speed, operating the pairof fans 114 a second speed, and operating the pair of fans 116 at athird speed, wherein the first speed is greater than the second speedand the second speed is greater than the third speed. Reducing the speedof the pair of fans 114 may further reduce noise when, for example, theposition of the pair of fans 114 partially axially overlaps with thepair of fans 112. In another aspect, automated distribution of torquecan be accomplished by operating the pair of fans 112 at a first speed,operating the pair of fans 114 at a second speed, and operating the pairof fans 116 at a third speed, wherein the second speed is greater thanthe first speed and the first speed is greater than the third speed.Reducing the speed of the pair of fans 112 relative to the pair of fans114 further reduces the noise generated by the pair of fans 116 bereducing the amount of turbulent air generated by the pair of fans 112.In this configuration, the pair of fans 114 may need to be operated at ahigher speed to provide the needed amount of thrust.

In some aspects, flight control computer 140 receives data from sensor162 to monitor an amount of noise generated by aircraft 100. Sensor 162can be a microphone, a seismometer, or the like for measuring a noise orvibration level. Flight control computer 140 may use a feedback loop tocontrol a speed of the pair of fans 116 based upon data received fromsensor 162. In some aspects, flight control system 130 may includemultiple sensors 162. For example, each fan of the pair of fans 116 mayhave its own sensor 162 positioned proximal thereto. In some aspects,each fan of the pairs of fans 112, 114, 116 may have its own sensor 162.Flight control computer 140 may, via a feedback loop, make reductions tothe power levels of the pair of fans 116 while at the same timebalancing the amount of total thrust needed to maintain flight byadjusting power levels to the pairs of fans 112, 114.

FIGS. 4A and 4B are flows diagram illustrating a method 200 forcontrolling a noise level of aircraft 100 according to aspects of thedisclosure. In some aspects, steps of method 200 are performedautomatically by flight control system 130 upon transitioning toairplane mode from helicopter mode. Method 200 begins at step 202. Instep 202 aircraft 100 transitions to airplane mode. After aircraft 100has transitioned to airplane mode, method 200 proceeds to step 204.

In step 204, a noise of level of aircraft 100 is measured. For thepurposes of this disclosure, noise level is used to include both soundlevels and/or vibration levels. For example, sensor 162 measures a noiselevel associated with the pair of fans 116. In some aspects, two sensors162 are used, with one proximal each fan of the pair of fans 116. Insome aspects, at least one sensor 162 is positioned proximal to each fan112, 114, 116 of aircraft 100. Method 200 then proceeds to step 206.

In step 206, flight control computer 140 compares the noise levelmeasured by sensor(s) 162 to a threshold noise level. The thresholdnoise level is a predetermined noise level that may be based upon noiserestriction requirements or may be specified by a user. If flightcontrol computer 140 determines that the measured noise level is greaterthan the threshold noise level, method 200 proceeds to step 208. Ifflight control computer 140 determines that the measured noise level isless than the threshold noise level and/or vibration, method 200 eitherreturns to step 204 or method 200 ends.

In step 208, flight control computer 140 reduces a speed of the pair offans 116 to reduce an amount of noise produced by the pair of fans 116.The speed of the pair of fans 116 is reduced by reducing power suppliedto rotor systems 122 of the pair of fans 116. In some aspects method 200then proceeds to step 210. FIG. 4B illustrates an optional aspect inwhich method 200 optionally returns to step 204 to again measure thenoise level to determine if the reduction in speed of the pair of fans116 has sufficiently reduced the noise level of aircraft 100.

In step 210, flight control computer 140 compares an amount of thrustproduced by fans 112, 114, and 116 to a threshold thrust level. Thethreshold thrust level is an amount of thrust needed to maintain adesired flight characteristic. If flight control computer 140 determinesthat the amount of total thrust being produced by fans 112, 114, and 116is less than the threshold thrust level, method 200 proceeds to step212. If flight control computer 140 determines that the amount of thrustbeing produced by fans 112, 114, and 116 is greater than or equal to thethreshold thrust level, method 200 proceeds to step 214 and method 200ends. Alternatively, method 200 can return to step 204 to continue tomonitor the noise level of aircraft 100.

In step 212, flight control computer 140 increases a speed of one orboth of the pair of fans 112 and the pair of fans 114 to provideadditional thrust to satisfy the threshold thrust level. The speed ofthe pair of fans 112 and/or the pair of fans 114 is increased byincreasing power supplied to rotor systems 122 of the pair of fans 112and the pair of fans 114. Method 200 then returns to step 204 to againmeasure the noise level.

Method 200 can be used to automatically tune fan speed to reduce noiselevels of aircraft 100 and to distribute torque across fans 112, 114,116. It will be appreciated by those having skill in the art that method200 could be used with aircraft having fewer than six fans and aircrafthaving more than six fans. As an alternative to method 200, flightcontrol computer 140 can simply reduce the power to the pair of fans 116without monitoring sensor data.

Referring now to FIG. 5, a schematic diagram of a general-purposeprocessor (e.g. electronic controller or computer) system 300 suitablefor implementing the aspects of this disclosure is shown. System 300includes processing component and/or processor 310 suitable forimplementing one or more aspects disclosed herein. In some aspects,flight control computer 140 and/or other electronic systems of aircraft100 may include one or more systems 300. In addition to processor 310(which may be referred to as a central processor unit or CPU), system300 might include network connectivity devices 320, random access memory(RAM) 330, read only memory (ROM) 340, secondary storage 350, andinput/output (I/O) devices 360. In some cases, some of these componentsmay not be present or may be combined in various combinations with oneanother or with other components not shown. These components might belocated in a single physical entity or in more than one physical entity.Any actions described herein as being taken by the processor 310 mightbe taken by the processor 310 alone or by the processor 310 inconjunction with one or more components shown or not shown in the system300. It will be appreciated that the data described herein can be storedin memory and/or in one or more databases.

Processor 310 executes instructions, codes, computer programs, orscripts that it might access from the network connectivity devices 320,RAM 330, ROM 340, or secondary storage 350 (which might include variousdisk-based systems such as hard disk, floppy disk, optical disk, orother drive). While only one processor 310 is shown, multiple processors310 may be present. Thus, while instructions may be discussed as beingexecuted by processor 310, the instructions may be executedsimultaneously, serially, or otherwise by one or multiple processors310. The processor 310 may be implemented as one or more CPU chipsand/or application specific integrated chips (ASICs).

The network connectivity devices 320 may take the form of modems, modembanks, Ethernet devices, universal serial bus (USB) interface devices,serial interfaces, token ring devices, fiber distributed data interface(FDDI) devices, wireless local area network (WLAN) devices, radiotransceiver devices such as code division multiple access (CDMA)devices, global system for mobile communications (GSM) radio transceiverdevices, worldwide interoperability for microwave access (WiMAX)devices, and/or other well-known devices for connecting to networks.These network connectivity devices 320 may enable the processor 310 tocommunicate with the Internet or one or more telecommunications networksor other networks from which the processor 310 might receive informationor to which the processor 310 might output information.

The network connectivity devices 320 might also include one or moretransceiver components 325 capable of transmitting and/or receiving datawirelessly in the form of electromagnetic waves, such as radio frequencysignals or microwave frequency signals. Alternatively, the data maypropagate in or on the surface of electrical conductors, in coaxialcables, in waveguides, in optical media such as optical fiber, or inother media. The transceiver component 325 might include separatereceiving and transmitting units or a single transceiver. Informationtransmitted or received by the transceiver 325 may include data that hasbeen processed by the processor 310 or instructions that are to beexecuted by processor 310. Such information may be received from andoutputted to a network in the form, for example, of a computer databaseband signal or signal embodied in a carrier wave. The data may beordered according to different sequences as may be desirable for eitherprocessing or generating the data or transmitting or receiving the data.The baseband signal, the signal embedded in the carrier wave, or othertypes of signals currently used or hereafter developed may be referredto as the transmission medium and may be generated according to severalmethods well known to one skilled in the art.

RAM 330 might be used to store volatile data and perhaps to storeinstructions that are executed by the processor 310. The ROM 340 is anon-volatile memory device that typically has a smaller memory capacitythan the memory capacity of the secondary storage 350. ROM 340 might beused to store instructions and perhaps data that are read duringexecution of the instructions. Access to both RAM 330 and ROM 340 istypically faster than to secondary storage 350. The secondary storage350 is typically comprised of one or more disk drives or tape drives andmight be used for non-volatile storage of data or as an over-flow datastorage device if RAM 330 is not large enough to hold all working data.Secondary storage 350 may be used to store programs or instructions thatare loaded into RAM 330 when such programs are selected for execution orinformation is needed.

The I/O devices 360 may include liquid crystal displays (LCDs),touchscreen displays, keyboards, keypads, switches, dials, mice, trackballs, voice recognizers, card readers, paper tape readers, printers,video monitors, transducers, sensors, or other well-known input oroutput devices. Also, transceiver 325 might be considered to be acomponent of the I/O devices 360 instead of or in addition to being acomponent of the network connectivity devices 320. Some or all of theI/O devices 360 may be substantially similar to various componentsdisclosed herein and/or may be components of any of flight controlsystem 130 and/or other electronic systems of aircraft 100.

Depending on the aspect, certain acts, events, or functions of any ofthe algorithms, methods, or processes described herein can be performedin a different sequence, can be added, merged, or left out altogether(e.g., not all described acts or events are necessary for the practiceof the algorithms, methods, or processes). Moreover, in certain aspects,acts or events can be performed concurrently, e.g., throughmulti-threaded processing, interrupt processing, or multiple processorsor processor cores or on other parallel architectures, rather thansequentially. Although certain computer-implemented tasks are describedas being performed by a particular entity, other aspects are possible inwhich these tasks are performed by a different entity.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain aspects include, while other aspects do notinclude, certain features, elements and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or more aspectsor that one or more aspects necessarily include logic for deciding, withor without author input or prompting, whether these features, elementsand/or states are included or are to be performed in any particularaspect.

The term “substantially” is defined as largely but not necessarilywholly what is specified (and includes what is specified; e.g.,substantially 90 degrees includes 90 degrees and substantially parallelincludes parallel), as understood by a person of ordinary skill in theart.

In any disclosed aspect, the terms “substantially,” “approximately,”“generally,” “generally in the range of,” and “about” may be substitutedwith “within [a percentage] of” what is specified, as understood by aperson of ordinary skill in the art. For example, within 1%, 2%, 3%, 5%,and 10% of what is specified herein.

While the above detailed description has shown, described, and pointedout novel features as applied to various aspects, it will be understoodthat various omissions, substitutions, and changes in the form anddetails of the devices or algorithms illustrated can be made withoutdeparting from the spirit of the disclosure. As will be recognized, theprocesses described herein can be embodied within a form that does notprovide all of the features and benefits set forth herein, as somefeatures can be used or practiced separately from others. The scope ofprotection is defined by the appended claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A method of reducing noise generated by atilt-rotor aircraft, the method comprising: transitioning the tilt-rotoraircraft from helicopter mode into an airplane mode; and reducing aspeed of a first pair of fans of the tilt-rotor aircraft to be less thana speed of a second pair of fans positioned in-line with the first pairof fans.
 2. The method of claim 1, wherein the tilt-rotor aircraftcomprises a third pair of fans.
 3. The method of claim 2, wherein thethird pair of fans is positioned out of line with the first pair offans.
 4. The method of claim 2, comprising setting a speed of the thirdpair of fans equal to the speed of the second pair of fans.
 5. Themethod of claim 2, comprising setting a speed of the third pair of fansto be greater than the speed of the second pair of fans.
 6. The methodof claim 2, comprising setting a speed of the third pair of fans to beless than the speed of the second pair of fans.
 7. The method of claim1, comprising: measuring, prior to the reducing the speed of the firstpair of fans, a noise level of the first pair of fans; and determiningif the noise level exceeds a threshold noise level.
 8. The method ofclaim 7, comprising: measuring, after the reducing the speed of thefirst pair of fans, the noise level of the first pair of fans;determining if the noise level is below the threshold noise level; andresponsive to a determination that the noise level is greater than thethreshold noise level, reducing the speed of the first pair of fans. 9.The method of claim 1, comprising: measuring, after the reducing thespeed of the first pair of fans, a total thrust produced by the firstand second pairs of fans; comparing the total thrust produced by thefirst and second pairs of fans to a threshold thrust level; andresponsive to a determination that the total thrust produced by thefirst and second pairs of fans is less than the threshold thrust level,increasing a thrust level of the second pair of fans.
 10. The method ofclaim 1, wherein the reducing the speed of the first pair of fans isdone automatically by a flight control system of the tilt-rotoraircraft.
 11. A flight control system configured to reduce a noise levelof a tilt-rotor aircraft, the flight control system comprising: a flightcontrol computer comprising a processor; a propulsion systemcommunicatively coupled to the flight control computer; a first pair offans and a second pair of fans communicatively coupled with the flightcontrol computer and the propulsion system; wherein the processor isoperable to implement a method comprising: transitioning the tilt-rotoraircraft into an airplane mode from a helicopter mode; and reducing aspeed of the first pair of fans to be less than a speed of the secondpair of fans that are positioned in-line with the first pair of fans.12. The flight control system of claim 11, comprising: a sensorcommunicatively coupled to the flight control computer and configured tomeasure a noise level of the first pair of fans.
 13. The flight controlsystem of claim 11, wherein the tilt-rotor aircraft comprises a thirdpair of fans communicatively coupled with the flight control computerand the propulsion system.
 14. The flight control system of claim 13,wherein the third pair of fans are positioned out of line with the firstpair of fans.
 15. The flight control system of claim 13, wherein themethod comprises setting a speed of the third pair of fans to be equalto the speed of the second pair of fans.
 16. The flight control systemof claim 13, wherein the method comprises setting a speed of the thirdpair of fans to be greater than the speed of the second pair of fans.17. The flight control system of claim 13, wherein the method comprisessetting a speed of the third pair of fans to be less than the speed ofthe second pair of fans.
 18. The flight control system of claim 13,wherein the method comprises: measuring, prior to the reducing the speedof the first pair of fans, a noise level of the first pair of fans; anddetermining if the noise level exceeds a threshold noise level.
 19. Theflight control system of claim 18, wherein the method comprises:measuring, after the reducing the speed of the first pair of fans, thenoise level of the first pair of fans; determining if the noise level isbelow the threshold noise level; and responsive to a determination thatthe noise level is greater than the threshold noise level, reducing thespeed of the first pair of fans.
 20. The flight control system of claim18, wherein the method comprises: measuring, after the reducing thespeed of the first pair of fans, a total thrust produced by the firstand second pairs of fans; comparing the total thrust produced by thefirst and second pairs of fans to a threshold thrust level; andresponsive to a determination that the total thrust produced by thefirst and second pairs of fans is less than the threshold thrust level,increasing a thrust level of the second pair of fans.